1. Field of the Invention
The present invention relates to a scanner and a method of scanning an image on an original, wherein an optical sensor having sensor elements for each of three basic colors is used to generate pixel values of the scanned image, each pixel value representing an optical density of a pixel of the image. Thus, a monochrome, i.e. a grey scale or black and white (B/W) image of the original is obtained using a color scanner.
2. Description of Background Art
In reduction type scanners, an optical sensor with one or more linear CCD (charge coupled device) arrays and a lens are arranged to focus lines of an original onto the CCD line arrays. The scanner usually has one light source emitting white light.
In a color scanner, for example, there are at least three CCD line arrays having color filters for red, green, and blue light, respectively, thus forming sensor elements for the basic colors of red, green, and blue light. Typically, the sensor elements for different colors have different light sensitivities. When a grey scale image is to be obtained from a color original, the original is scanned, and the signals from the sensor elements for red, green, and blue light are mathematically mixed in order to calculate the grey intensity from the RGB values of each pixel of the image. Hence, the grey level of a pixel is a weighted average of the RGB levels, wherein the weight factors are defined such that the hue and saturation information is eliminated. This will be called the conventional scan mode in the following.
The scan speed of a reduction type scanner is limited by the maximal clock speed of the CCD. This is a profound limitation in view of a demand for high scan resolutions. In order to obtain a higher scan speed, it is known to additionally provide a panchromatic sensor element for B/W data. Thus, the grey intensity of a color original can be scanned using only the panchromatic sensor element. However, providing an extra panchromatic sensor element is costly.
Moreover, a B/W original could be read using only a single sensor element of a color sensor, e.g. the sensor element for green light, when the sensor element for green light has the highest sensitivity.
U.S. Pat. No. 5,923,447 discloses a method of reading a color original to obtain a B/W image using at least two of the R, G, and B light receiving elements of an optical sensor. A B/W image could not be received by using only the sensor element for, e.g. green light, because a green part of the original would then be recognized as white. By using adding means to add the signals output from the at least two of the R, G, and B light receiving elements for each pixel, a black-white judging means is enabled to correctly judge, based on the added result, whether the pixel is to be represented as black or white. However, this method is not applicable when a grey scale image is to be obtained from a color original.